Field of the Invention
The present invention relates in general to methods of fabricating microelectronic structures using dual-layer, light-sensitive developer-soluble bottom anti-reflective coatings, especially suited for high-NA 193-nm lithography.
Description of the Prior Art
Integrated circuit manufacturers are consistently seeking to maximize substrate wafer sizes and minimize device feature dimensions in order to improve yield and increase on-chip computing power. The demand for high density and small feature size has been constantly pushing photolithography technology to its limits. The delay of non-optical lithography technology has required the extension of existing techniques and technologies.
The minimum feature that may be patterned using an optical lithography process is determined by the following equation (Rayleigh's theory):
  W  =                    k        1            N        ⁢          λ      A      where W is the resolution, k1 is the resolution factor (a constant), λ is the wavelength of the exposure radiation, and NA is the numerical aperture of the exposure tool. Rayleigh's theory indicates that an exposure tool with a short wavelength and large numerical aperture will produce better pattern resolution. More recently, trends toward improving current 193 nm photolithography processes have included: 1) increasing the actual numerical aperture (NA) of the imaging lens to >0.9, thereby increasing resolution; and 2) using immersion lithography to increase the effective NA to ≧1.0, increasing resolution as well as depth of focus. Use of high-NA lithography at the 65 nm half-pitch node and immersion lithography enables k1 to remain above 0.3. Using imaging tools with high-NA capabilities (>1.0) by themselves or in combination with immersion provides a method to achieve higher resolution of patterns with smaller critical dimension and higher density.
However, high-NA lithography has its own set of problems. For example, high numerical apertures cause light to diffract at high angles. In addition, use of high-NA lenses, especially in immersion lithography, results in a wide range of angles of incidence. This deviation from normal incidence causes increased reflectance at the resist-air and resist-substrate interfaces. Increased reflectance in turn causes an increase in both standing waves and CD swing. One approach to this problem has been to use a bottom anti-reflective coating applied to the substrate beneath the photoresist layer to reduce substrate reflectance. Top anti-reflective coatings, applied on top of the photoresist layer, have also been used to reduce reflectance at the top surface of the photoresist. It has been found that single layer bottom anti-reflective coatings and top anti-reflective coatings do not provide adequate reflectance control in high-NA lithography, especially when the substrate includes topography.
There is a need in the art for bottom anti-reflective coating strategies that offer increased reflection control for high-NA lithography including immersion lithography, especially over topography, that provides effective reduction in CD swing, as well as improved exposure latitudes, depth of focus, and line edge roughness. There is also a need for bottom anti-reflective coatings that are suitable for high-NA lithography, but are removable with photoresist developers.